[PATCH] netlink: fix (theoretical) overrun in message iteration

[Vegard Nossum]

See commit 1045b03e07d85f3545118510a587035536030c1c for a detailed
explanation of why this patch is necessary.

In short, nlmsg_next() can make "remaining" go negative, and the
remaining >= sizeof(...) comparison will promote "remaining" to an
unsigned type, which means that the expression will evaluate to
true for negative numbers, even though it was not intended.

I put "theoretical" in the title because I have no evidence that
this can actually happen, but I suspect that a crafted netlink
packet can trigger some badness.

Note that the last test, which seemingly has the exact same
problem (also true for nla_ok()), is perfectly OK, since we
already know that remaining is positive.

Cc: Thomas Graf <tgraf@xxxxxxx>
Signed-off-by: Vegard Nossum <vegard.nossum@xxxxxxxxx>
---
 include/net/netlink.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/include/net/netlink.h b/include/net/netlink.h
index 3643bbb..13dd525 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -332,7 +332,7 @@ static inline int nlmsg_attrlen(const struct nlmsghdr *nlh, int hdrlen)
  */
 static inline int nlmsg_ok(const struct nlmsghdr *nlh, int remaining)
 {
-	return (remaining >= sizeof(struct nlmsghdr) &&
+	return (remaining >= (int) sizeof(struct nlmsghdr) &&
 		nlh->nlmsg_len >= sizeof(struct nlmsghdr) &&
 		nlh->nlmsg_len <= remaining);
 }
-- 
1.5.6.5

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